The present invention relates to an emergency supply system for an emergency flight control of a high performance aircraft of unstable design and to a power transmission unit for such an emergency supply system.
The draft designs of single-seat or two-seat fighter planes for the near future (e.g., the Hunter 90) focus on improvement of the flying properties (maneuverability, climbing ability, slow flight, etc.). Apart from the choice of propulsion mechanism and material (e.g., composite fiber materials) the aerodynamic concept is of great importance, the trend being toward the so-called canard type with aerodynamically unstable design. The canard type has numerous control surfaces specifically with a view to mobility, but this requires a voluminous drive system with the disadvantage of great vulnerability. Also with a view to great mobility, the lift and control surfaces are arranged, in consideration of the center of gravity of the aircraft, in such a way that the aircraft acts in an aerodynamically unstable manner.
So that it will nevertheless remain easy to control by the pilot, it must be artificially restabilized by appropriate control systems, the vulnerability being increased, of course, by these systems also. Now in order to achieve a flying safety comparable with simpler designs despite the greater vulnerability, at least the energy systems required for emergency flight control must be as failureproof as possible. This involves the electric on-board system for power generation/transmission as well as for the transmission of control commands, measuring and regulating signals, etc., and one or more hydraulic systems for the direct actuation of the control surfaces. A common measure, which serves not only for power increase but also for safety, is the installation of two propulsion mechanisms. Each engine then has its own equipment carrier, so that at least the drive groups for the hydraulic system (pump) and for the electric system (generator) are provided in duplicate (redundance). Known also is the provision of engine-independent turbines, which in an emergency are admitted with propulsion gases by a decomposition of a single-component fuel (e.g., hydrazine), these turbines serving to restart the engines or directly for the - brief - drive of important groups. Such single-component fuel systems are, however, cost-intensive, expensive and not quite without danger. There is no problem with the installation of non-rechargeable expendable batteries, which for several minutes can deliver a relatively high power. In modern two-jet fighter planes already in use, with two mutually independent hydraulic systems, it is known practice to actuate the control surfaces required for emergency flight control simultaneously with both hydraulic systems (parallel systems). To be able to transmit power from one hydraulic system to the other also directly, a power transmission unit is installed, which consists of two hydraulic pumps/motors with input/output shafts coupled together, so that one group can be operated as a motor, the other as a pump. This power transmission unit can be coupled in and out via 2/2 distributing valves. In addition, there are several emergency hydraulic pumps which can be driven selectively via a hydraulic drive or via an electric motor for each. These pumps, motors and single-component fuel systems - provided in addition to the power transmission unit and the main hydraulic pumps -are expensive and cost-intensive and in normal flight are only a heavy, useless ballast.
It is known from DEP 31 11 722 to provide various engine-indepedent energy sources, for example, a relative wind turbine, which drives one generator and one hydraulic pump. The possibility has also been mentioned, of driving a generator and a pump with an additional accessory gas turbine and supplying an electric motor from a battery, which, in turn, drives a pump. Additional groups of this nature, intended exclusively for emergency operations, undoubtedly increase the degree of fail-safety. Under normal operating conditions they are, however, only additional ballast and increase the difficulty and expense of maintaining the aircraft.
In terms of electric energy sources it should be stated that, aside from engine-dependent and engine-independent generators, rechargeable accumulators as well as non-rechargeable emergency batteries are used. Known hydraulic systems are described in the Greman Magazine "Oelhydraulik und Pneumatik" 10 (1966) Nr. 1. pages 10 to 14. According to this reference, it is customary with modern transport aircraft, to provide for all important functions two parallel-connected essentially identical hydraulic systems, which under normal operating conditions operate together, but which, however, are layed out in such a way, that in an emergency, one system suffices for operating the craft, with the operating processes in view of the reduced power, proceeding, for example, only at half the speed. The two hydraulic systems can be entirely separated from each other. They can, however, also be connected in terms of energy, for example, with a connection valve. Also, the hydraulic systems can be coupled to the electric on-board system by providing in each of the hydraulic systems, in addition to the engine-driven pump, an accessory pump with electromotor drive for emergencies. Moreover, each of the two hydraulic systems can be divided into two or more subsystems, connected in series and via an excess pressure valve. In doing this, the less important secondary subsystem is supplied with pressure oil only when in the more important primary subsystem the full oil pressure decreases. In case of leaks in the primary subsystem the secondary subsystem remains non-operational. The rudder functions are, corresponding to their degree of importance to the ability to fly, assigned to the subsystems.
In summary, it can be stated that the known systems permit in emergencies the supply of power to the hydraulic systems, between the hydraulic systems as well as to the electric on-board system, for which engine-dependent and independent energy sources are considered. In order to do this, relatively expensive, heavy, and maintenance-intensive accessory groups are required, which reduce the payload of the aircraft.